FOREPOLING METHOD OF TUNNELING PDF

i. A method of advancing a mine working or tunnel in loose, caving, or watery ground, such as quicksand, by driving sharp-pointed poles, timbers, sections of. 2 Sep needle beam method of tunneling ppt For easy office pdf filter the excavation and temporary support of the station tunnel, a forepoling method. The following methods are generally employed for tunnelling in hard rocks. Full face method . Forepoling is an old method of tunnelling through soft ground.

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In hard rock, the drill holes for the bench forepoljng driven at the same time as the removal of the muck. Other details regarding tunnelling in such a soil are the same as for sand.

Requires instant support throughout the excavation. The method, however, requires the construction of an additional tunnel and therefore the time and cost of construction are higher as compared to the methods described before. In the case of soft rock, the selection of the method of tunnelling depends upon the following important factors.

The needle beam is placed horizontally with its front end supported on the drift and the rear end supported on a vertical post resting on the lining of the tunnel.

This method normally involves the digging of two tunnels, namely, a pilot tunnel and a main tunnel. Proper care should be taken to ensure that the material does not block the propelling jacks and other equipment.

The airlock is an airtight cylindrical steel chamber with a door at each end opening inwards. Tunnelling may be basically divided into two main groups. Finally, the work of lining the arch is meyhod out and further excavation is done. The sidewalls and face of the tunnel can do without support for one or two hours, but the roof can last only a few minutes.

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Excavation of the tunnel section can be carried out without support for small lengths ranging from 2 to 5 m. During excavation, the rail requires support at the sidewalls and the roofs depending upon methox type of soil. The method is also known as the German method. In this method Fig. The work is carried on till the springing level is reached.

The process is repeated as the work progresses. The side drift, however, has the advantage that it permits the use of timber to support the roof. The process of widening is continued till mehod reaches the springing level. A number of drill holes are provided all around the drift and these are filled up with explosives and ignited so that the size of the drift expands meethod become equal to the required cross section of the tunnel. The cross section of the pilot tunnel usually measures about 2.

Examples include sand stone, hard clay, etc. Some of the important methods of tunnelling in soft rock are described in the following sections. A drift is a small tunnel measuring 3 m x 3 m, which is driven into the rock and whose section tynneling widened in subsequent processes till it equates that of the tunnel.

The types of ground which are generally encountered in the field are detailed in Table Shield tunnelling is forepolung preferred in such cases. The vertical posts now occupy the entire roof level.

A series of holes fodepoling 10 mm to 40 mm in diameter with about mm centre-to-centre distance are then drilled into the rock, preferably in two rows. The bulk head shield is used in this case. The drift is supported by laggings, caps, and two vertical posts. The excavation is carried out below these poles, which are supported by vertical posts.

The posts supporting the drift can then be removed and tunnelling work continued further in a similar manner. The pilot tunnel offers the following advantages.

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A shield is a movable frame that is used to support the face of a tunnel. Forepooling determination of this value depends more on experience than on technical considerations.

Definition of forepoling

In the linear plate method Fig. This method requires good quality timber as well as simultaneous and frequent shifting from place to place. Examples include firm clay, gravel, and dry earth. Forepoling is an old method of tunnelling through soft ground. Examples include damp sand, soft earth, and certain types of gravel. Poles are then inserted at the top of the frame up. It basically consists of a cutting edge, a skin plate in the form of a shell structure, and a hood of jacks, ring girders, stiffening steel plates, ports as well as port doors, and a tail.

The full face method is normally selected for small tunnels whose dimensions do not exceed 3 m. In this case, tunnelling is of the open type.

Methods of Tunnelling

Tunnels constructed using the shield method usually have a circular section because of the mehtod considerations.

The various methods of shield tunnelling through different types of soils are enumerated in Table The drift is subsequently widened. In this method, a frame is prepared in the shape of the letter A, placed near the face of the tunnel, and covered with suitable planks. Next the muck is removed before repeating the process of drilling holes. The equipment for tunnelling consists of a bulk head, which is an airtight diaphragm with an airlock. This method is used for long tunnels, particularly those at great depths, where the walls of the excavation may yield under the msthod of the cover.